The present disclosure relates generally to methods for coating a surface and more particularly is directed to a ceramic coating process.
Conventional thermal spray processes use powders with particle size ranging from 10 to 100 μm. They result in coatings that mainly present micrometer-sized features. The features are the result of the lamellae formed by the impact of the particles onto the substrate. These lamellae are a few micrometers (μm) thick with a diameter ranging from a few tens to a few hundreds of micrometers.
Coatings can obtain enhanced properties by utilization of nanometer-sized particles as compared to micrometer-sized ones. Reducing the structure scale down to a nanometer allows, increasing strength, improving toughness and while reducing apparent density, elastic modulus, and apparent thermal conductivity, among other improvements.
One of the major drawbacks in processing nanometer-sized particles by thermal spraying is the difficulty in injecting them in the core of the high enthalpy flow, since the particle injection force has to be of the same order as that imparted by the gas flow. Additionally, conveying fine ceramic powders less than 10 μm through traditional thermal spray powder feeders is challenging due to effects of humidity and electrostatic charging of the powder during transport. Both of these mechanisms, and others, can lead to the clogging of the powder feed line to the thermal spray torch. Thus, it is not practically possible to inject particles with sizes below 5-10 μm.
Possibilities exist to circumvent these drawbacks through the use of a carrier medium by which powders can be brought to the thermal spray torch and injected into the high energy gas flow of the thermal spray torch.